Method and control for maintaining optimum performance of HVDC power transmission systems at rectifier end during A. C. system fault

摘要

A new and improved control subassembly for HVDC power converters and method of its use during transient faults affecting the A. C. system employed in the HVDC power transmission system to assure the presence of adequate firing and commutating voltages across the respective HVDC thyristor valves during selected intervals of the alternating current periods of the respective phases. The HVDC power converters used in the system include gating circuits for gating-on respective ones of the thyristor valves during selected intervals of the alternating current periods of the respective phases. The gating circuits include equidistant- spaced gate firing pulse generating circuits for supplying equidistant- spaced gating-on firing pulses to the gate drives for the respective HVDC thyristor valves of the HVDC bridge power converter during normal operation of the converter in the presence of symetrically-shaped A. C. system voltage waveforms. The gating circuit further includes firing mode switch circuits connected intermediate the equidistant-spaced gate firing pulse generators and the gate drives for the respective HVDC thyristor valves. The firing mode switch circuits are supplied with the outputs from an A. C. system monitor for reducing and monitoring the voltage waveshapes of the A. C. system voltage for detecting the points at which the respective phase voltages of the multi- phase alternating current system pass through zero voltage value in a direction which enables the respective associated HVDC valves for conduction and for deriving output commutating voltage indicating signals representative of this occurrence. A minimum valve voltage detector also is coupled to the A. C. system for sensing that any of the A. C. system phase voltages are below a predetermined minimum safe level required to assure safe operation of the HVDC power converters and derives an output minimum valve voltage alarm signal under conditions where the minimum safe level has not been attained by the voltage across any one of the HVDC thyristor valves. The minimum valve voltage alarm signal is utilized to control the firing mode switch circuits in such a manner that the commutating voltage waveform signals for the respective phases from the output of the A. C. system monitor after suitable signal processing is used to drive the gate drives for the respective HVDC thyristor valves in the presence of a minimum valve voltage alarm signal in place of the normal equidistant-spaced gating-on firing pulses under conditions where asymmetries occur in the A. C. system voltage waveforms due to transient disturbances affecting the system. Upon termination of the fault, the minimum valve voltage alarm signal terminates and the system returns to normal with the HVDC thyristor valves being gated-on by the equidistant- spaced firing pulses.